arm64: set ATTR_CONTIGUOUS on the DMAP's L2 blocks

On systems configured with 16KB pages, this change creates 1GB page
mappings in the direct map where possible. Previously, the largest page
size that was used to implement the direct map was 32MB. Similarly, on
systems configured with 4KB pages, this change creates 32MB page
mappings, instead of 2MB, in the direct map where 1GB is too large.

Implement demotion on L2C (32MB/1GB) page mappings within the DMAP.

Update sysctl vm.pmap.kernel_maps to report on L2C page mappings.

Reviewed by: markj
Tested by: gallatin, Eliot Solomon <ehs3@rice.edu>
Differential Revision: https://reviews.freebsd.org/D45224